We are defining the role of the specific duodenal calcium binding protein in intestinal calcium transport by studying calcium absorption in the presence (normal rat) and virtual absence (diabetic rat) of calcium binding protein. Calcium absorption in the duodenum of the diabetic rat is half that observed in control rats. Ileal absorption of calcium is the same in both diabetic and control rats. Associated with decreased duodenal calcium absorption is a marked decrease in the specific duodenal calcium binding protein in the diabetic rat. We are comparing calcium absorption and secretion at various calcium concentrations in control and diabetic rats by in vivo and in vitro techniques. The differences in duodenal absorption and secretion of calcium in control and diabetic rats define the role of calcium binding protein in calcium transport. The cause for deficiency in calcium binding protein in the diabetic rat is being sought by (1) examining the effects of vitamin D and its metabolites on duodenal calcium binding protein; (2) comparing synthesis rate and turnover of calcium binding protein in control and diabetic rats; and (3) studying the effects of hormones on the deficiency of calcium binding protein in the diabetic rat.